In multibeam color cathode ray tubes, it is conventional practice to utilize an internal multi-opening component, such as an aperture mask, in the forward portion of the tube. Such is suitably supported within the tube envelope in spaced relationship to an adjacent patterned cathodoluminescent screen disposed on the interior surface of the tube display area or viewing panel. Common types of patterned screens utilized with the above-mentioned multi-opening component may be comprised of a multitude of dot triads, or stripes of different color emitting phosphors disposed as an array on the viewing panel in a predetermined repetitive sequence in related registry with the apertures in the multi-opening mask. The alignment relationship of the openings with the adjacent elements of the screen pattern is a requisite for the subsequent phosphor excitation by the specific electron beams directed thereto.
The term "aperture" or "opening" as used herein, is given a broad connotation to include those discretely formed mask openings that are substantially round, ovate, or slot-like shapings, some versions of which may be delineated by a supported array of spatially-related wires or strips.
The multi-opening portion of the mask-frame assembly is usually a substantially domed structure which is affixed to a substantially rigid metallic perimetrical supporting or framing member. This essential supporting member is usually a one-piece drawn structure representing an integration of top, bottom, and side elements transitionally combined at angular corner regions to provide a continuous sidewall portion, shaped to mate with the domed multi-opening portion which is bonded thereto to complete the mask-frame assembly. The structure of the framing member manifests a substantially L-shaped cross-sectional configuration by reason of a ledge portion instanding from the sidewall portion. The assembly is then positioned, by a plurality of support means, within the tube viewing panel in close proximity to the patterned cathodoluminescent screen disposed therein.
By the inherent constructional shaping of the framing member, structurally weaker areas are evidenced in the corners and immediately adjacent regions. This structural weakness is the result of the designed reduction of material in those areas and the drawn shaping which tends to somewhat further reduce the material thickness thereat. Thus, as a result of these weakening factors, the stresses induced by the drawing procedure often impart a slight twist or warp to the corner regions. When the multi-opening mask portion is affixed to the framing member, in the mask-to-frame assembly procedure, mechanical jigging is utilized to seat the framing member into a planar position prior to consummation of the bonding. However, when the mask-frame assembly is subsequently lehr-heated during the glass panel-to-funnel sealing procedure, the constraining influences of the mask portion are lessened, whereupon the stressing forces in the framing member tend to again assume prominence and thereby produce undesirable corner-oriented twisting. This becomes more pronounced as the size of the framing member is increased. Any warpage or twisting of the mask-frame assembly within the tube causes misalignment of the mask openings with the respective phosphor elements of the patterned screen, thereby deleteriously affecting both resolution and color purity of the screen display.
Prior art techniques to overcome the aforementioned twisting action include the incorporation of straight strength ribs in the corner regions, such being oriented substantially perpendicular to a diagonal bisector therethrough, and as such, cut across the flow lines of the corner material. Additionally, the usual deep shaping of the ends of such ribs tends to unduly stretch the material in the corners, thereby introducing additional stresses which, in themselves, tend to aggravate warpage when released during tube processing.
An advancement in the art is taught by Peter G. Puhak in U.S. patent application Ser. No. 241,176, filed Mar. 6, 1981, now U.S. Pat. No. 4,362,963 and assigned to the assignee of this invention. Puhak discloses the use of a substantially crescent-shaped strengthening indentation formed in each corner of the framing member. Such indentations have open concave edges facing the central opening of the framing member, and rounded and sloped termini. These crescent shapings have markedly strengthened the framing members employed in the smaller sized tubes, such as the 12 and 15 inch screen sizes. But, in the larger tubes, such as the 19 and 25 inch sizes, wherein increased leverage of the framing side members is evidenced, it has been found that additional strengthening of the mask-framing member is desired.